Yes, this is what the iSub was. However, it doesn't work with Tiger
Apple apparently are working on it, but the last update bought no changes.
Other than that, there are none that I am aware of, and there's no way of using the headphones connection at the same time as the built in speakers to drive a subwoofer as far as I'm aware.
TBH, the built in speakers are great, but you'd be far better off just getting a discreet set of computer speakers...